1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device, and more particularly, to a flash type EEPROM (Electrically Erasable Programmable Read Only Memory, and hereafter called a flash memory). In this flash memory, stored information (data) is electrically and collectively erasable, and then the stored information can be rewritten.
2. Description of the Related Art
Recently, in accordance with popularization of computers, word processors, and the like, a plurality of semiconductor memory devices, typically nonvolatile semiconductor memory devices, e.g., flash memory, used in such information processors, have been developed and produced.
The flash memory, which is one kind of nonvolatile semiconductor memory device, can be made programmable by the user. Further, the flash memory can be rewritten by electrically and collectively erasing the stored data, and then by programming. Note, in the flash memory, it is known that an over-erase phenomenon (which will be explained in detail later) may be caused by repeatedly carrying out write/erase operations over a long period of use.
Therefore, in a normal read operation, even though a cell is in a non-selection state, a gate voltage of the over-erased and non-selected cell is increased until the gate voltage exceeds a threshold voltage thereof, and the over-erased and non-selected cell may be erroneously selected, so that erroneous data is read out and a reliability of the flash memory may become low.